crest factors, such as a pulse train with low duty cycle,

should have an average time constant chosen to be at

least ten times the signal period.

increases the settling time for a step change in the input

signal level. Figure 3 shows the relationship between

settle to within a given percent of the change in RMS

level, is set by the averaging time constant, which varies

approximately 2:1 between increasing and decreasing

input signals. For example, increasing input signals

require 2.3 time constants to settle to within 1%, and 4.6

time constants for decreasing signals levels.

In addition, the settling time also varies with input signal

levels, increasing as the input signal is reduced, and

decreasing as the input is increased as shown in

Figures 6a and 6b.

Using Post Filters

ripple and improves the overall settling time. The value

mum DC error at the lowest frequency of interest. The

post filter is used to remove excess output ripple.

Figures 7, 8, and 9 give recommended filter connec-

tions and values for both the MX536A and MX636.

Table 1 lists the number of time constants required for

the RMS section to settle to within different percentages

of the final value for a step change in the input signal.

Decibel Output (dB)

The dB output of the MX536A/MX636 originates in the

squarer/divider section and works well over a 60dB

range. The connection for dB measurements is shown

in Figure 10. The dB output has a temperature drift of

0.03dB/°C, and in some applications may need to be

compensated. Figure 10 shows a compensation

scheme. The amplifier can be used to scale the output

for a particular application. The values used in Figure

10 give an output of +100mV/dB.

True RMS-to-DC Converters

_______________________________________________________________________________________

9

10

0

1

2.5

1m

100m

10

1

5

7.5

RMS INPUT LEVEL (V)

10m

MX536A

Figure 6a. MX536A Settling Time vs. Input Level

10

0

1

2.5

1m

100m

1

5

7.5

RMS INPUT LEVEL (V)

10m

MX636

Figure 6b. MX636 Settling Time vs. Input Level

Settling Time

to Within

Stated % of

New RMS

Level

1%

0.1%

0.01%

4.6

τ/4.6τ

6.9

τ/6.9τ

9.2

τ/9.2τ

Table 1. Number of RC Time Constants

(

τ) Required for MX536A/MX636 RMS

Converters to Settle to Within Stated % of

Final Value

FOR

DECREASING

AMPLITUDES

Basic Formulas

PARAMETERS

Note:

(

τ) Settling Times for Linear RC Filter

4.6

τ/2.0τ

6.9

τ/3.1τ

9.2

τ/4.2τ

FOR

INCREASING

AMPLITUDES

∆V